1. Field of the Invention
The present invention relates to a solid polyelectrolyte-type fuel cell.
2. Discussion of the Background
A solid polyelectrolyte-type fuel cell is considered to be a hopeful, small-sized lightweight power source for vehicles and other devices which uses hydrogen and oxygen as fuel. The cell comprises an ion-exchangeable, solid polyelectrolyte membrane, and a positive-electrode and a negative electrode disposed to be in contact with both sides of the membrane. The fuel hydrogen is electrochemically oxidized at the negative electrode to give protons and electrons. The protons pass through the polyelectrolyte membrane toward the positive electrode, which is fed oxygen. Electrons, having been formed at the negative electrode, travel to the positive electrode, where the protons and the electrons react with oxygen to form water.
The solid polyelectrolyte-type cell can operate at low temperatures and is small, while producing a high power output density. Therefore, many studies have been made on these cells for use as a power source for vehicles. Generally used in the cell is a sulfonic acid group-containing perfluorocarbon polymer membrane (e.g., NAFION, trade name of DuPont Co.; ACIPLEX, trade name of Asahi Chemical Co.) or the like as the polyelectrolyte membrane. However, the conventional fuel cell is not still satisfactory, because its output is not high enough.
In order to increase the output of the cell, the hydrogen ion conductivity of the polyelectrolyte membrane therein must be increased to thereby lower the internal resistance of the cell. For this, the concentration of the ion-exchanging groups (for example, sulfonic acid groups) existing in the polyelectrolyte membrane may be increased and the thickness of the membrane may be reduced. However, too great an increase in the ion-exchanging group concentration in the membrane results in an increase in the water content of the membrane, and is therefore problematic in that the positive electrode at which water is formed through cell reaction becomes too wet, lowering the cell output.
On the other hand, the reduction in the thickness of the membrane is also problematic in that the mechanical strength of the membrane is reduced, and that the amount of fuel, (hydrogen gas and oxygen gas) passing through the membrane is increased, lowering the cell-output efficiency.
One prior technique is disclosed in Japanese Patent Application Laid-open (JP-A) Hei-6-231781. A cation-exchanging membrane of a laminate, composed of at least two layers of a sulfonic acid group-containing perfluorocarbon polymer, in which the layers each have a different water content, is used as the solid electrolyte membrane, so that the membrane may have low electric resistance. In the solid polyelectrolyte-type fuel cell disclosed therein, a plurality of polymer films, each having a different water content, are so laminated to construct the polyelectrolyte membrane that their water content varies to increase from the positive electrode side to the negative electrode side.
Another prior technique is disclosed in JP-A Hei-6-231782. The polyelectrolyte-type fuel cell disclosed therein comprises a laminate membrane of at least two, sulfonic acid group-containing perfluorocarbon polymer films, each having a different water content, in which the water content of the polymer film facing the positive electrode is made larger than that of the polymer film facing the negative electrode, in order that the laminate membrane may have low electric resistance.
However, the former, in which the water content of the laminate membrane greatly varies around the lamination boundaries, suffers from the problem that some stress is generated around the boundaries, thereby lowering the mechanical durability of the membrane. In addition, in the former, the discontinuous variation in the water content of the laminate membrane interferes with efficient back diffusion of water from the positive electrode that compensates for the reduction in the water content of the membrane adjacent to the negative electrode, whereby the cell can not produce a large output. On the other hand, in the latter, the positive electrode becomes too wet and the negative electrode has high resistance. In this, therefore, the increase in the cell output could not be attained.
In order to increase the output of the fuel cell of that type, the hydrogen ion conductivity of the polyelectrolyte membrane in the cell must be high and the internal resistance of the membrane must be small. The hydrogen ion conductivity and the internal resistance of the membrane are significantly influenced by the amount of ion-exchanging groups (for example, sulfonic acid groups) existing in the membrane and also by the water content of the membrane. Specifically, membranes having a higher ion-exchanging group content and a higher water content have a higher degree of hydrogen ion conductivity.
In the negative electrode side of the fuel cell, hydrogen ions derived from hydrogen gas pass through the polyelectrolyte membrane and move toward the side of the positive electrode. In the fuel cell, therefore, the water content of the membrane adjacent to the negative electrode is lowered, thereby causing the reduction in the cell output. On the other hand, water is formed through cell reaction in the membrane adjacent to the positive electrode so that excess water exists therein. As a result, therefore, it is presumed that such excess water will cover the catalyst, and interfere with gas diffusion, thereby causing a reduction in the cell output.
Still another prior technique is disclosed in JP-A Hei-6-231783. In the polyelectrolyte-type fuel cell disclosed therein, the solid polyelectrolyte membrane is of a cation exchanging membrane having a laminate structure of at least three layers of a sulfonic acid group-containing perfluorocarbon polymer each having a different water content, in which the water content of the polymer film layers adjacent to the positive electrode and to the negative electrode is larger than that of the interlayer polymer film.
In the three prior techniques noted above, the solid polyelectrolyte membrane is a sulfonic acid group-containing perfluorocarbon polymer. In those, a plurality of such polymer layers each having a different water content are laminated to construct the solid polyelectrolyte membrane, in order to increase the cell output.
However, the starting materials, such as tetrafluoroethylene, 2-fluorosulfonyl-perfluoroethyl vinyl ether and others for the polymer, are extremely expensive, and the high price of these materials is fixed, and are unlikely to significantly fall in the future. In addition, the polymer is further problematic in that it requires complicated polymerization steps to be followed by the final step of sheeting it into thin films. As a result, if the polymer is used in producing fuel cells for electric cars, the price of each fuel cell produced is high and will be equal to the price of the car itself. For the same reasons as above, the polymer could not be used in producing fuel cells for leisure appliances. Therefore, using the polymer in producing practical fuel cells is not practicable at present.
Given the situation, the present invention is to provide a high-output, solid polyelectrolyte-type fuel cell which is characterized in that the water content of the polyelectrolyte membrane in the cell is not uniform, but rather continuously varies in the direction of the thickness of the membrane in such a manner that the water content is highest in the side of the membrane adjacent to the negative electrode, and is lowest in the side thereof adjacent to the positive electrode. As a result, in the cell of the invention, the surface of the membrane repels water and promotes the back-diffusion of water from the positive electrode, which compensates for the water content loss in the membrane adjacent to the negative electrode, and therefore the positive electrode is prevented from becoming to wet and the catalytic action in the cell is promoted to increase the cell output.
The invention also provides a high-output, solid electrolyte-type fuel cell in which the polyelectrolyte membrane is of a laminate of at least two layers each having a different water content so as to optimize the water retentiveness of the membrane that faces the positive electrode and the negative electrode. This is based on the technical idea of the applicant""s own invention in a prior patent application of JP-A Hei-9-102322, hereby incorporated by reference. As in the prior application, an ethylene-tetrafluoroethylene copolymer (ETFE) film base is exposed to radiation, then grafted with styrene at the resulting radicals, and thereafter sulfonylated with chlorosulfonic acid and hydrolyzed with an alkali or acid to thereby introduce sulfonic acid groups into the grafted copolymer. The solid-polyelectrolyte composition thus produced is based on the ETFE film, and is produced through irradiation followed by grafting.